Federal and state governments have imposed increasingly strict regulations over the years governing the levels of hydrocarbon (HC), carbon monoxide (CO) and nitrogen oxide (NOx) pollutants that a motor vehicle may emit to the atmosphere.
Vehicle emission performance is measured by a vehicle dynamometer test schedule determined by the Federal, and sometimes State Governments. Other countries have different emission tests based on their particular needs. During the vehicle emissions test, harmful materials such as hydrocarbons, HC, carbon dioxide, CO, and nitrous oxides, NOx, are measured at the vehicles tailpipe in terms of grams per mile or other units capable of being used in a standardized test.
One approach to reducing the emissions of these pollutants involves the use of a catalytic converter. Placed within the exhaust gas stream between the exhaust manifold of the engine and the muffler, the catalytic converter is one of the several emissions control devices typically found on a motor vehicle.
Catalytic converters consist of a ceramic substrate with many channels for exhaust flow to pass through. These channels are coated with a wash-coat and catalyst such as Platinum, Palladium, and Rhodium. The catalyst coated substrate is then wrapped with either an intumescent mat material such as 3M Company's Interam 100 or a non-intumescent mat material such as Interam 1101 HT to adjust for manufacturing tolerances, and to retain the catalyst in its steel container. Catalytic converters speed chemical reactions without taking part in the reactions. To function with significant efficiency a catalytic converter must be warmed by engine exhaust flow to its minimum operating temperature, this is normally a temperature of greater than about 350 degrees C., for automotive catalytic converters.
The catalytic converter is essentially a reaction chamber that contains an oxidation catalyst, typically in the form of one or more monolithic substrates, coated with a high surface area ceramic wash-coat and one or more precious metals such as Platinum, Palladium or Rhodium. When the engine is running, the exhaust gases from the exhaust manifold flow through the converter and pass heat to those composite materials housed within it. Once heated to a suitably high temperature, the composite materials convert a large percentage of the pollutants in the passing exhaust gases to carbon dioxide (CO2), water (H2O) and other benign substances. Until the converter is brought up to operating temperature, however, its composite materials do not operate as effectively. As is well known to individuals skilled in the related arts, the catalytic converter is particularly inefficient when it is at its coolest, just after the engine is started cold.
A large percentage of a vehicles total cold start, HC emissions occur during the time period while the catalytic converter is warming up to operating temperature.
To reduce the emissions of harmful materials, particularly during initial cold-start-up, several attempts have been made to reduce cold start emissions, for example: the catalytic converter has been moved as close to the engine as possible. In cases where the entire converter could not be moved close enough to the engine, a smaller warm-up converter is often used ahead of a second under-floor converter. In addition, catalytic converter improvements such as improved catalysts, and high-cell-density ceramic substrates with very thin walls that require less heat energy to reach operating temperature have been employed to reduce cold start emissions. Also, engine improvements such as electronic fuel injection, with closed loop-air-fuel ratio control, have also been employed to reduce tailpipe emissions.
As mentioned above, one approach that has been proposed to reduce the emission of HC, CO and NOx pollutants while the exhaust system is cold is to use a second catalytic converter, often referred to as a warm-up converter. The warm-up converter would be small in size and located near the engine so that it could warm-up quickly. It would employ composite materials (i.e., a substrate, an oxidation catalyst and catalytic material coating) specially formulated to reach operating temperature quickly, thereby quickly rendering the warm-up converter capable of efficiently converting the pollutants in the exhaust gas. This is significant, as most of the pollutants are produced during the first minute or two after the engine is started. Until the engine and exhaust system have warmed to the point at which the conventional converter is operating more effectively, the exhaust gases during this “warm-up period” would be routed into the warm-up converter to remove the pollutants from the exhaust gases.
Given its proximity to the engine, and small size the warm-up converter will generally not be able to withstand continuous exposure to certain harmful poisons carried by the exhaust gases without a significant loss in performance. In particular, engine oil that may have been burned in the combustion chambers will be carried away by the exhaust gases into the exhaust system. Certain compounds in the oil, such as zinc-dithio-phosphate, will gradually coat the catalyst in the warm-up converter and reduce or significantly reduce its effectiveness. Prolonged exposure to the exhaust gases will therefore prematurely degrade the composite materials inside the warm-up converter.
A solution to this problem would be to strategically place an exhaust control valve within the exhaust system. Controlled by the engine control module (ECM) or other control component with feedback from a suitable sensor, the exhaust control valve can be automatically opened to allow exhaust gases to flow through the warm-up converter during the warm-up period and closed to prevent such flow afterward. By switching the flow of the exhaust gases away from the warm-up converter after the warm-up period, the exhaust control valve would then protect it from the relatively high temperatures and the harmful compounds carried by the exhaust gases. This tends to keep the warm-up converter free of poisons and highly effective during the warm-up period.
After the warm-up period, the conventional converter due to its large size best treats the HC, CO and NO x pollutants. The large size of the conventional converter makes it more resistant to such poisoning. Due to the increased cost and questionable reliability of exhaust systems with valves, vehicle manufactures have not used them to maintain warm-up converter performance.
Despite improvements vehicle manufacturers have made they are continually challenged by the reductions in allowable grams per mile of tail pipe emissions, mandated by the Federal Government, and certain State Governments such as California with high levels of air borne emissions in their cities, and to do this in a low cost, reliable manner. Accordingly, it is desirable to improve the cold start emissions of a vehicle by providing methods and apparatus for meeting the mandated emissions standards at minimum cost, and without reducing reliability.